This article series will be dedicated to helping readers understand Einstein’s theory of special relativity. We will be using Russell Stannard’s introduction to the topic. It helps to imagine sitting in a train carriage, waiting at the station. Looking out the window, you observe a second train next to yours. Your train begins to move and you pass the other train. Finally, its last carriage disappears from your view. This allows you to see the station itself disappearing into the distance as you leave it behind.
Suppose, however, you realize that it is you who are moving rather than the train disappearing in the distance. The other train is just sitting there. When it comes to being in steady, uniform motion in a straight line, it is impossible to tell whether you are moving or if it is another body that is moving. We normally know we are moving in a car because you feel the car turning and going over bumps, but this is not the case in an airplane, flying at a steady speed.
When you are inside a vessel that is moving at a steady rate of speed, this is known as having an inertial frame of reference. This is an allusion to Newton’s law of inertia, according to which an object does not change its seed or direction unless acted upon by an unbalanced force. An object on a table, for example, will just sit there unless you manually move it.
Looking out the airplane window does not tell you that the airplane is moving either. It could be that the earth is moving. Indeed, the earth is moving. It rotates around the sun. The sun itself is orbiting the center of the Milky Way. The Milky Way itself is moving around in a cluster of other galaxies. In all of the aforementioned cases, the movements are relative. The earth and the plane are moving relative to one another. Everyone has an inertial frame of reference. The rules of nature govern everyone in uniform steady motion. This means that anyone can consider any other object as being at rest while the other person is moving. This is known as the principle of relativity.
This principle precedes its supposed discovery by Einstein. Indeed, it was Galileo who first conceived it. Einstein’s revolution consisted in a special application and extension of this principle. According to Maxwell’s laws of electromagnetism, it is possible to calculate the speed of light, c, in a vacuum, provided you have knowledge of the strengths of electric and magnetic forces. It ma sound strange to think of light having a speed, since its appearance seems instantaneous when you turn on the lights.
It does take a definite amount of time for light to flood the room when you turn on the switch, however. The speed of light in a vacuum is 299,792.458 kilometres per second. This is only slightly different in air. Light behaves in a particular strange manner. When it comes to the speed of most bodies, we can imagine a shell being fired from a warship, where an observer on the shore can expect the speed of the ship to be added to the shell’s speed if fired in the forward direction and subtracted if being fired to the rear.
This is not how light works, however. This behavior was measured in a CERN laboratory in Geneva in 1964, thanks to observation of very small subatomic particles. These are known as “neutral pions.” They travel at 0.99975 c, and decay with the emission of two light pulses. Both travel at the speed of light within the measurement accuracy of 0.1 percent. In any case, the speed of light does not depend on the speed of its source.